This invention relates to decoding of signals that are encoded with a predictive coding schema.
The subject matter disclosed herein is related to
U.S. Pat. No. 5,134,477, issued Jul. 28, 1992 [rel. app. 1], PA1 U.S. Pat. No. 5,243,419, issued Sep. 7, 1993 [rel. app. 2], PA1 U.S. patent application Ser. No. 07/817,206 filed Jan. 6, 1992 [rel. app. 3], and PA1 copending U.S. patent application Ser. No. 07/919,792 filed Jul. 27, 1992, titled "An Improved Adaptive Leak HDTV Encoder" [rel. app. 4].
In related copending application [rel. app. 1] an encoding schema is disclosed together with a corresponding decoder. The disclosed encoding and decoding is proposed for a terrestrial high definition television (HDTV) environment in the U.S., but the disclosed principles, of course, have a much broader applicability.
To briefly describe the disclosed arrangement, the input signal to be encoded is, for example, a video signal that comprises a concatenation of signal segments that correspond to signals that make up an image frame. That signal is evaluated in a forward estimation portion of the encoder and various parameters are developed by that evaluation. Thereafter, with the aid of the developed parameters, the signal is encoded and thereby substantially compressed, buffered, modulated and finally transmitted to the decoder (e.g., an HDTV receiver). Some of the parameters developed in the forward estimation portion of the encoder are also transmitted to the decoder, including motion vectors and image mean signals. In accordance with the teaching of the related copending application [rel. app. 1], the signals transmitted by the encoding portion comprise scale factor signals, quantized vector signals and vector codebook identifier signals.
In carrying out the differential encoding process, the encoder must be aware of the signals that the target receiver has, in order for the encoder to take account of whatever encoding/decoding errors are introduced by the process and cannot be avoided. To that end, the encoder includes a frame buffer that is populated with signals that are derived from the encoded signals created by the encoder.
At the decoder end, i.e., at the receiver, the received signal are decoded with the aid of a frame buffer that maintains the previously decoded signals. The frame buffer within the decoder corresponds to the aforementioned frame buffer within the encoder.
One problem with this approach, when strictly executed, is that errors introduced into the signal following the encoding process show up in the decoder's frame buffer and do not disappear. To ameliorate this potential problem, the disclosed encoder arrangement introduces a signal leak. That is, the encoder does not encode the difference between the cur-rent frame and a prediction of the current frame derived from the previous frame that is stored in the frame buffer. Rather, the signal that is encoded is the difference between the current frame and only a fraction of the prediction frame. In effect, a portion of the current frame is encoded, because it is not nullified by the prediction frame. That portion, called the "leak", is controlled in accordance with image characteristics and other parameters that relate to the encoder's operation. The actual control is realized by providing what effectively is a multiplication circuit responsive to the output of the frame buffer, which circuit multiplies its applied signals by a fraction. The leak signal is also transmitted to the decoder.
In the related application filed herewith [rel. app. 4], it is also disclosed that the encoder includes an output buffer. In order to maintain a constant delay between the encoder's input signal and the decoder's output signal, it is important to know the level of fullness of the encoder's output buffer (e.g., in terms of the number of image frames stored therein). Accordingly, related application [rel. app. 4] discloses an encoder that also transmits an output buffer fullness control signal.
The arrangement described above works well in that an injected perturbation, such as transmission noise entering the frame buffer, is removed within a number of image frames by virtue of the multiplication-by-a-fraction process that occurs at the output of the frame buffer. Still, such perturbations are not welcome, and whenever there is a priori information that such a perturbation is about to manifest itself, it would be beneficial to overcome it quickly.